mm, vmscan: unlock page while waiting on writeback

This is merely a politeness: I've not found that shrink_page_list()
leads to deadlock with the page it holds locked across
wait_on_page_writeback(); but nevertheless, why hold others off by
keeping the page locked there?

And while we're at it: remove the mistaken "not " from the commentary on
this Case 3 (and a distracting blank line from Case 2, if I may).

Signed-off-by: Hugh Dickins <hughd@google.com>
Acked-by: Michal Hocko <mhocko@suse.com>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/mm/vmscan.c b/mm/vmscan.c
index 8276a3a615ca0049515affe9adbd4a0480ce8c4a..2d978b28a410b25df1acde351630dee387efbbe5 100644 (file)
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -985,7 +985,7 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                 *    __GFP_IO|__GFP_FS for this reason); but more thought
                 *    would probably show more reasons.
                 *
-                * 3) Legacy memcg encounters a page that is not already marked
+                * 3) Legacy memcg encounters a page that is already marked
                 *    PageReclaim. memcg does not have any dirty pages
                 *    throttling so we could easily OOM just because too many
                 *    pages are in writeback and there is nothing else to
@@ -1015,12 +1015,15 @@ static unsigned long shrink_page_list(struct list_head *page_list,
                                 */
                                SetPageReclaim(page);
                                nr_writeback++;
-
                                goto keep_locked;
 
                        /* Case 3 above */
                        } else {
+                               unlock_page(page);
                                wait_on_page_writeback(page);
+                               /* then go back and try same page again */
+                               list_add_tail(&page->lru, page_list);
+                               continue;
                        }
                }